The present invention relates to a method of fabricating a semiconductor device, and more particularly, a semiconductor device having on the same semiconductor substrate a DMOS with high driving capacity and a resistor circuit using a highly precise polycrystalline silicon resistor.
A conventional fabrication method is described using FIGS. 2(a)-2(b). After a gate oxide film 102 having a thin film thickness and a field oxide film 103 having a thick film thickness are formed on a semiconductor substrate 101 of a first conduction of conductivity type, a gate electrode 104 of a MOS transistor is formed of first polycrystalline silicon, and further, oxidization is carried out to form an oxide film 105, which is shown in FIG. 2(a). Next, as shown in FIG. 2(b), in an attempt to form a so-called body region (an impurity region 108 of a second conduction type) of the DMOS, impurity of the second conduction type is doped in the semiconductor substrate 101 of the first conduction type by ion implantation with the gate electrode 104 being the mask, and is diffused by thermal annealing. Then, as shown in FIG. 2(c), second polycrystalline silicon is adhered, impurity is doped, and thereafter, patterning is carried out to form a resistor 106. Next, as shown in FIG. 2(d), first impurity having high concentration is heavily doped in the semiconductor substrate 101, the second impurity region 108, and a part of the second polycrystalline silicon resistor 106 to form a source and a drain 110 and 110 of the MOS transistor and a junction with wiring metal after the resistor. In particular, if the conductivity type of the impurity is the same as that of the second polycrystalline silicon resistor, as shown in FIG. 2(d), a photoresist film 112 is patterned by photolithography. After that, with the photoresist 112 and the gate electrode 104 serving as a mask, the source 110 and the drain 110 of the MOS, the second polycrystalline silicon resistor 106 and a high concentration region 111 can be formed at the same time. The high concentration region 111 becomes an ohmic contact for attaching a wiring layer.
However, with the conventional fabrication method, in order to make high the punch-through withstanding pressure between the source and the drain of the DMOS, sufficient diffusion in the body region must be carried out, and nevertheless, there is a problem that, in this case, the effective L length of the DMOS becomes longer and thus, the sufficient driving capacity of the DMOS is not obtained. Further, while the resistor is formed using the second polycrystalline silicon, in patterning it, the etching is isotropic such that the etched residue, that is, stringer, of the second polycrystalline silicon resistor is not formed on the gate electrode side walls, and, there is also a problem that, here, since side etching is carried out, the precision of the value of resistance of the polycrystalline silicon resistor is made low.
An object of the present invention is to solve the above problems and to provide a fabrication method for forming on the same substrate a DMOS with sufficient withstanding pressure and high driving capacity and a polycrystalline silicon resistor with high precision of the value of resistance.